1. Field of the Invention
The present invention relates to a cathode ray tube (CRT) used in a color television or a high-definition monitor television and further to an electron gun used in an electron beam exposure device or the like that utilizes a converged electron beam. In particular, the present invention relates to a field-emission electron source used in an electron gun of a highly bright CRT requiring a high current density operation, and an image display apparatus using the same.
2. Description of Related Art
In recent years, with the advent of thin-type displays such as liquid crystal displays or plasma displays, the flat display market has been growing rapidly, though CRT displays still hold an edge in price and performance for application to home televisions about 32 inch diagonal in size.
Furthermore, terrestrial digital broadcasting was newly introduced at full scale in 2003, and there has been a drastic change in the technologies of television displays. With televisions and their surroundings making a transition to a digital system, displays have been required strongly to have high-resolution performance.
However, the television technology that has been used widely so far might not be able to respond to such a demand sufficiently. An electron gun is used in a television as a main portion for displaying an image, and its performance is closely related to the resolution performance.
By increasing a current density of a cathode used in the electron gun, it becomes possible to reduce an effective area of the cathode, thereby improving the resolution performance. Although various technological improvements on a thermal cathode material that is currently used as the cathode of the electron gun have been made to increase the current density, such improvements have come close to their physical limits and no more dramatic increase in the current density can be expected.
A cathode in an electron gun for digital broadcasting, which has been proceeding toward a practical use in recent years, requires about 6 to 10 times as large a current density as a conventional thermal cathode. Accordingly, there are increasing expectations for a cold cathode as a technology for achieving a considerable increase in the current density.
This cold cathode is generally manufactured by using a semiconductor process. Since this process is advantageous in that a cathode having a minute structure on a sub-micron order or smaller can be integrated at a high density, the current density can be increased. Therefore, this cold cathode has been applied to products such as field-emission display apparatuses or the like.
In general, a refractory metal (high-melting-point metal) such as molybdenum often is used as a material for the cold cathode. After the completion of CRT manufacturing process, the vacuum level inside the CRT usually is about 10−4 Pa owing to constraints in the manufacturing processes and the structure of the CRT. When the cold cathode is operated at a current density of about 10 A/cm2 under such a vacuum environment, the following problem arises.
Inside the CRT, there are various kinds of residual gases that have been generated in the manufacturing process. It is known that oxygen (O) and carbon (C) among the constituent elements of the residual gases temporarily adhere to an emitter surface or change a composition of the emitter surface, thereby lowering the emission performance of the cold cathode.
For the above-mentioned object, Japanese Patent No. 2718144 discloses a concept regarding stabilization of an emission current by arranging, on a surface of a cathode, a chemically-stable resistance material having a low work function. A configuration of the conventional example will be described below by referring to FIG. 6.
FIG. 6 is a cross-sectional view to show a configuration of a conventional field-emission electron source 90.
On a conical tungsten cold cathode base 92, a film 82 of La2O3 as one of the low work function oxides is coated to a thickness of about 10 nanometers, thereby forming a field-emission cold cathode 83. In the vicinity, a lead electrode 93 having a through hole 95 with a diameter of about 1 μm is formed on an insulating layer 94 applied on a substrate 96. When a voltage of about 60 V is applied between the cold cathode base 92 and the lead electrode 93, electrons are emitted from the surface of the cold cathode base 92.
When the voltage was raised to 80 V, an emission electron current of 1 μA was obtained. With respect to the change of the emission electron current over time, fluctuation of the emission electron current was within 5% regardless of the vacuum level of 1×10−7 Torr. A field-emission cold cathode based on this system can provide a relatively stable operation in comparison with a conventional cold cathode having no La2O3 film, as the conventional cold cathode has a fluctuation of the emission electron current ranging from 30% to 40%.
The above effect is obtained due to a negative feedback from the La2O3 resistance film coated on the electrode surface. More specifically, the internal resistance of the La2O3 film prevents the electron emission from concentrating at a point, but the electrons are emitted from the entire surface of the sharpened top portion of the cold cathode. Moreover, the La2O3 film is stable with respect to the residual gas, and furthermore, an operation at a low voltage serves to decrease damage caused by the sputtering.
However, experimental results of studies by the inventors revealed that the above-mentioned conventional method can cause a problem as mentioned below.
Though JP 2718144 has no specific description about a method of forming a La2O3 resistance film, in many cases, a vacuum deposition method used for a process of manufacturing a semiconductor or a plasma sputtering that uses an argon (Ar) gas can be applied for forming a thin film of about 10 nanometers in thickness.
When such a film formation process is used for coating a La2O3 film 82 about 10 nanometers in thickness on a surface of a cold cathode base 92 so as to form a field-emission cold cathode 83, the La2O3 film 82 is applied partially on the surface of the insulating layer 94 at an opening in the lead electrode 93 as well as on the surface of the cold cathode base 92. The La2O3 film 82 formed on the surface of the insulating layer 94 will degrade the withstand voltage between the cold cathode base 92 and the lead electrode 93.
When a voltage of about 60 V is applied between the cold cathode base 92 and the lead electrode 93 in this state, a leakage current will occur between the cold cathode base 92 and the lead electrode 93, and this can prevent application of a normal voltage. This problem will degrade a stable field-emission characteristic.
Use of the La2O3 film 82 having an internal resistance is advantageous in that a comparatively stable operation is available regarding a current emission. However, due to the rise in the cathode surface potential caused by the internal resistance, an effective voltage between the cold cathode base 92 and the lead electrode 93 is decreased, resulting in a disadvantage, that is, an increase in the operation voltage.
The stabilization method using the internal resistance also is referred to as a ballast effect caused by a load resistance. Since the stabilization effect provided by increased internal resistance and the rise in the effective voltage are in a trade-off relationship, the stabilization has been difficult to optimize.
In a silicon minute structure cold cathode that includes a silicon substrate as a cold cathode base and that has the top portion sharpened by thermal oxidation, the top portion generally has a radius of curvature uniformly controlled to a level of several nanometers or less. When a La2O3 film having a thickness of about 10 nanometers is coated on the cathode surface of the silicon minute structure cold cathode according to a conventional method, the radius of curvature of the top portion of the cathode is decreased before the coating step. The radius of curvature can be multiplied occasionally by several dozens. Since the radius of curvature of the top portion of the cathode can have a great influence on the field-emission characteristic in light of the operation principle, the field-emission characteristic may deteriorate considerably.